Bimetallic Cooperativity of a Ferrocene-based Iridium NHC Complex in Water Oxidation Catalysis: A New Frontier for Efficient Oxygen Evolution.

Abstract

Bimetallic catalysts have gained attention as promising contenders, owing to the synergistic interaction between two distinct metal centers. In this study, we present two N-heterocyclic carbene iridium(III) pentamethylcyclopentadienyl complexes [Cp*Ir(fcpyNHC)Cl]PF6 (1) and [Cp*Ir(pyNHC)Cl]PF6 (2) where 1 includes a ferrocene moiety acting as a bimetallic complex. Using ceric ammonium nitrate as a sacrificial oxidant, both complexes were tested for water oxidation. Complex 2 achieved a maximum turnover number (TONmax) of 3240 and a turnover frequency (TOFmax) of 231 min-1. In comparison, complex 1 demonstrated nearly double the activity with a TONmax of 6047 and TOFmax of 431 min-1 compared to 2, which was attributed to the cooperative effect of the catalyst in water oxidation reaction. This bimetallic Fe^Ir catalyst (1) exhibited outstanding catalytic efficiency for oxygen evolution from water at ambient conditions. We identified a proposed FeIII^IrIV intermediate experimentally via UV-Vis spectroscopy and XPS study. Theoretically, this intermediate was more stable by 7.84 kcal/mol than the traditional FeII^IrV electromer intermediate. This delineates the pronounced bimetallic cooperative participation of both Fe and Ir metal centres for better activity.